Added 'string.length' method recently documented.
Added 'string.length' method recently documented.

According to this issue (https://github.com/jquense/yup/issues/418), this method was not documented until recently.
dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,borisyankov/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped